Once the stuff of science fiction, a human mission to Mars may be becoming more feasible, and a new report in the 31 May issue of Science provides insight into the relevant radiation hazards.

Exposure to radiation has long been known to be a problem for participants in deep space missions. Because these missions can take years, they expose anything or anyone on board to high energy particles called Galactic Cosmic Rays (GCRs), and also to lower-energy Solar Energetic Particles (SEPs). Characterizing the radiation that spacecraft destined for Mars or other deep space locations absorb is essential for improving the safety of these vehicles.

Now, a report by Cary Zeitlin at Southwest Research Institute and colleagues details the radiation environment aboard the Mars Science Laboratory (MSL)--the spacecraft that carried the Curiosity rover to Martian soil in 2011 and 2012. Previous measurements of the radiation environment in deep space were made with unshielded instruments, which is not ideal in terms of assessing the potential hazard for humans, who will only travel to deep space in vehicles with shields.

"Data from our study are different because the radiation detector we used, Radiation Assessment Detector, or RAD, was under quite a bit of shielding," explained Dr. Zeitlin. The Mars Science Laboratory, from which Zeitlin and his colleagues took their readings, was protected by a complex shield far deeper than that on the Apollo spacecraft, for example. "Thus our measurement is the first of its kind."

For most of the MSL's 253-day journey to Mars, which lasted from November 26, 2011, to August 6, 2012, RAD made detailed measurements of the energetic particle radiation environment in the MSL interior, outputting a rich dataset. "I'm perpetually excited to see RAD work so well," Zeitlin said.

Because the shielding provided by the MSL is roughly similar to shielding likely to be used for future human trips to deep space, the RAD-reported doses onboard are realistic. Based on these measurements, and assuming similar shields and timing in the solar cycle, as well as a trip duration of 180 days (NASA's typical estimate for a fast outbound flight to Mars), Zeitlin and colleagues report that the radiation dose an astronaut traveling to and from the "Red Planet" would experience would represent a large fraction of his or her accepted lifetime limit.

Time spent on the Martian surface would add even more.

Because the work of Zeitlin and his team considers just the radiation exposure on the trip to and from Mars, he said the team's next step is to continue the radiation measurements from Curiosity as it travels over the Martian surface. "Publishing these results will give the research community additional information to use in evaluating mission scenarios."

Making this data available is especially critical in light of some of the Mars landing scenarios considered by NASA. "In some of them," Zeitlin explained, "the sequence of events is the trip to Mars, followed by something like 500 days on the surface, and then the trip back. The time on the surface is the longest part."

Prior to the work by Zeitlin and his team, there had been several calculations of the radiation exposure an astronaut on a Mars mission would receive. These predictions were made using models that incorporated educated guesses about the shielding distribution of the vessels used, as well as assumptions about the state of the solar cycle, both of which affect radiation exposures.

Along these lines, Dr. Zeitlin explained that he was surprised by the solar cycle state during MSL's cruise to Mars.

"Based on predictions about solar cycle progression from a few years ago," he said, "we'd have expected to be at or near solar maximum in late 2011 and the first half of 2012." Solar maximum is associated with a strong solar magnetic field, which suppresses the intensity of GCRs. Instead, the current solar maximum has been very weak so far, with relatively little solar activity. "And because of the weak solar maximum," Zeitlin explained, "the flux of GCRs during the trip to Mars was on the high side."

Even so, the results from this study are representative of a trip to Mars under conditions of low to moderate solar activity and fall within the range of previously modeled predictions for radiation exposure on a mission to Mars.

Dr. Zeitlin added a cautionary note to those who want to use RAD results to make definitive pronouncements about the feasibility of a human mission to Mars. "Radiation exposure at the level we measured is right at the edge, or possibly over the edge of what is considered acceptable in terms of career exposure limits defined by NASA and other space agencies. Those limits depend on our understanding of the health risks associated with exposure to cosmic radiation, and at present, that understanding is quite limited."

A community of researchers is hard at work to better quantify radiation risks.

Zeitlin said that it is very exciting to be part of the MSL Science Team and the larger mission to better understand the climate, geology, and mineralogy of Mars. "We have a front-row seat for results as they come in from other instruments," he said. "In addition to our own work, we get to see and hear about all the great work the other instrument teams are doing."

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The report by Zeitlin et al. was supported by NASA and the German Aerospace Center.

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